Students will learn how to classify objects and consider the bases for classification of naturally occurring objects by developing and evaluating their own schemes for rocks. Ohio Science Model Goal 1: The Nature of Science.

Proficiency Outcomes

4th Grade Test:

1. Create and/or use categories to organize a set of objects, organisms or phenomena.
4. Use a simple key to distinguish between objects.

6th Grade Test:

1. Use a simple key to classify objects, organisms, and/or phenomena.
10. Identify simple patterns in physical phenomena.
12. Identify characteristics and/or patterns in rocks and soil.

Materials Needed

  • “Buckets” of rocks: 1 bucket per 3-4 students; at least 10-12 rocks per bucket (a variety (various sizes, colors, grain sizes, presence or absence of layering or other features; i.e. a variety of visible characteristics) of rocks should be included, but no specific rock types are necessary)
  • Paper and pencil to record ideas
  • Optional: hand lens, magnifier, or simple reflected light microscope (10x or 15x magnification)

Developing a Classification for Rocks

They are everywhere - whether we are walking along a beach, planting flowers in the soil, hiking a stream bed, or skiing down a mountain - rocks! What are they, why are they important, and how do we make sense out of the diversity of rocks which make up our planet?


1. Divide students into groups of 3 or 4, and arrange them so they are facing each other to maximize discussion. Provide each group with a container of rocks.

Humans are by nature classifiers. We try to make sense out of our world by lumping things into groups based on, what? Tell the students to separate their rocks into a small number (their choice) of groups based on any criterion/criteria they choose. The goal is for each group to divide their rocks into a meaningful number of groups, by developing their own classification scheme based on any criteria they choose.

Circulate among the groups as they work. Some will need a bit of prodding - ask them questions, but don’t provide any answers! Respond to their questions with questions, e.g., “Does that seem to work with this rock?” They will want to know if they are getting it “right,” so you will need to keep reminding them that whatever they come up with is “right” - they will likely try characteristics and then discard them a few times until they hit on one or more that “work” for their rocks.

2. Once the students in each group have agreed on a classification scheme, have them write a description of their scheme. What is the basis of their scheme and how is it applied in practice? This will include answering the following questions:

  1. What criterion/criteria did you use?
  2. How many rock groups did you end up with?
  3. What criteria did you try but discard? Why didn’t it/they seem to work?
  4. Why did the criterion/criteria you ultimately used seem to “work” well? What is it about these criteria that make them valuable in classifying rocks?
  5. Do you have one or more samples that just don’t seem to fit your scheme? Why do you think this is?
  6. On a separate sheet of paper, write a flow chart of your method: step 1, step 2, etc.

3. Groups exchange written flow charts of classification schemes. Each group then attempts to apply this other group’s scheme to their set of rocks. Each group write a critique of the classification scheme based on their relative success in applying it to their rocks. How well did it work? Did it result in a reasonable distribution of rocks into a reasonable number of groups?

4. Class discussion of classification schemes. List basics of each scheme on board and lead discussion to determine what was similar about the schemes. All were observational. This can lead into discussion/investigation of modes of formation of rocks.

* Optional: Have each group write a definition of “rock.” Pretend you are Webster. Based on the fact that all these samples are rocks, how is rock defined?

{rock: a naturally formed coherent aggregate; i.e. must contain more than one “piece;” a rock can, therefore, be made of only one type of mineral, but there must be more than one piece, or grain of that mineral; more commonly rocks contain more than one type of mineral; a “rogue” rock is obsidian, a volcanic glass. Since obsidian is glass, it is not an aggregate, but as it is formed naturally during volcanism, and therefore in a “rock-forming” environment, we do consider it to be a rock.}

How do Geologists Observe, Describe, and Interpret Rocks?

Geologists divide, or classify, all rocks into one of three families based upon similarities in how the rocks form. The three families are:

  • igneous: formed by solidification (crystallization = mineral growth from liquid) from molten rock, e.g. magma or lava
  • sedimentary: formed by accumulation of sediments (broken pieces of pre-existing rocks) or precipitates (from surface, ground, or ocean water) at and near Earth’s surface
  • metamorphic: formed by recrystallization (mineral growth from solid minerals) of minerals in pre-existing rocks owing to changing conditions (heat, pressure, chemically active fluids) within Earth’s lithosphere.

5. Ask students: What observations that you made before can be used to interpret how the rocks may have formed?

Help guide the students toward the following...

For igneous: crystallized texture (how the grains fit together): means that crystals are interlocking, cannot be broken apart without strong force (eg. hammer); coarse grained (i.e. can see grains with unaided eye): cooled slowly from magma inside Earth’s crust; fine grained (need magnification to see most grains): cooled relatively quickly, probably after eruption from a volcano.

For sedimentary: clastic texture: clasts are broken pieces of other rocks; “clasts” are what you see in streams and rivers, and along beaches; they include pebbles and sand grains, and larger and smaller clasts. All of these can become cemented together to form a new, sedimentary rock. Chemically precipitated sedimentary rocks may be more interlocking and fine grained, and therefore more difficult to recognize. However, if they are marine in origin they often contain fossils, the remains of, for example, the shells of marine creatures, which shows they are indeed sedimentary in formation.

For metamorphic: recrystallized texture: with the unaided eye, this will also appear as an interlocking texture, like the igneous rocks. However, since these rocks form by solid-state mineral growth under the influence of heat and pressure, the rocks commonly have minerals arranged in layers (particularly if they contain mica minerals), or exhibit other patterns such as folding of layers, and compositional banding, whereby similar composition, and therefore color, minerals will form separate layers within the rock, giving it a striped appearance.

Note on rock color: color gives a general indication of composition. Rocks are made up of minerals, which are the individual crystalline “grains” which together make the rocks. Mineral color is a reflection of composition, i.e. the elements which make up the mineral; the color may come from the major elements in the mineral, meaning the elements that define that mineral, or the color may derive from impurity elements which got “caught” in the mineral as it grew. As such, color is not very useful in determining which rock family a rock belongs to, as many minerals are found in all three families of rocks! For the silicate minerals (95% of the rock-forming minerals), a green or black color indicates magnesium and/or iron as part of the mineral; a light color indicates a lack of these two elements, and the presence of aluminum, potassium, or sodium as part of the mineral composition.

* Optional: Develop a flow chart of rock classification which takes observations and modes of formation into account.

6. Give each student a rock and ask them to decide whether it is igneous, sedimentary, or metamorphic. Have them write a paragraph which includes their observations and reasons for classifying the rock as they do.

* Optional: Use the Internet to visit sites which display rock and thin section photos and illustrations of the rock cycle. Most of the sites which show photographs of rocks and thin sections for microscopic viewing have been developed by professors for their classes, and so vary in quality, updating, and availability. A good general resource for mineral and rock sites is the Geological Survey of Canada.

Rocks as Resources

To get the students thinking about the importance of rocks in their lives, ask them what we use rocks for. Have some good chunks available for demo: building stone, aggregate for road construction, rock salt for winter roads, for instance.

Minerals mined from rocks are used for everything that we can’t “grow!” For example:

  • Copper wire for electricity conduction: copper occurs naturally and is therefore a mineral!
  • Aluminum cans: lightweight yet strong; aluminum is obtained from an ore deposit called bauxite, which is dominated by the aluminum hydroxide mineral gibbsite.
  • Glass: made from quartz sand
  • Steel: the iron is obtained from iron-bearing minerals such as hematite.

*Optional: Rock classification practice can be adapted to dichotomous keys, which are commonly developed in these grade levels. Since it takes some practice to comfortably establish some rocks within the correct family based solely on macroscopic observation, I would suggest using dichotomous keys within each of the rock families.

Karen Fryer
Professor of Geology